The present invention generally relates to copying apparatuses and more particularly, to a copying apparatus which is capable of controlling density of a copied image by retractably projecting into an optical path a screen for improving gradation characteristics of the copied image.
Generally, in electrophotographic copying apparatuses, it is difficult to reproduce an image of medium density in the same manner as other image forming apparatuses such as a printer, etc Conventionally, in the copying apparatuses, it is known that if a screen having transparent and opaque portions formed by a pattern of lines, meshes or dots is placed in an optical path, gradation characteristics of a copied image are remarkably improved.
However, if the screen is placed in the optical path, the opaque portions of the screen are disposed in the optical path, thereby resulting in substantial reduction of quantity of light of an exposure lamp for attenuating surface voltage of a photosensitive member.
Meanwhile, FIG. 1 shows one example of a screen having a pattern of lines In FIG. 2, the curves (i) and (ii) show gradation characteristics in the case where the screen of FIG. 1 is disposed and is not disposed adjacent to the photosensitive member, respectively. It is seen from FIG. 2 that gradation characteristics are remarkably improved by using the screen when an original document has low density. However, if an original document having characters or drawings, for example, an original document having character A shown in FIG. 3a is copied by using the screen of FIG. 1, the copied image is undesirably formed with slits as shown in FIG. 3c. On the contrary, if the original document of FIG. 3a is copied without using the screen of FIG. 1, a proper image is obtained as shown in FIG. 3b. Therefore, it is desirable that the screen can be retractably projected into the optical path selectively according to original documents.
However, if the screen is merely retractably projected into the optical path, the following serious problem arises. Namely, generally, the known copying apparatuses have a control function to maintain quantity of light of the exposure lamp at a fixed level at all times in response to variations of external input voltage. Since quantity of light of the exposure lamp changes greatly at the time of projection of the screen into or retraction of the screen from the optical path as described above, control range of quantity of light of the exposure lamp should be made quite wide. Furthermore, such a screen mechanism is widely employed in color copying apparatuses in which gradation characteristics are especially vital. In the color copying apparatuses, since R (red), G (green) and B (blue) filters for separating colors of the original document, quantity of light of the exposure lamp is further reduced by these filters in addition to the screen, such an inconvenience is incurred that an excellent image cannot be obtained in some photosensitive members, for example, an OPC (organic photoconductor) type photosensitive member having low sensitivity generally.
Meanwhile, when the known copying apparatuses are changed over to a gradation display mode in which the screen is used and a standard mode in which the screen is not used, copying conditions change, thus offering the following problem. Namely, in order to obtain a proper or desired density of the copied image according to density of the original document, the known copying apparatuses are provided with an adjusting dial or an adjusting key switch for adjusting density of the copied image. However, since quantity of exposure light changes greatly between cases of presence and absence of the screen as described above, it is difficult to adjust density of the copied image to a proper or desired level in both the gradation display mode and the standard mode by using an identical adjustment range.